This invention relates to vacuum conveyors and particularly to such conveyors adapted to transfer printed sheets from a delivery conveyor to a sheet stacking apparatus.
Vacuum take-off conveyors are well known in the art as means to deliver an object from a delivery conveyor to a stacking area. One specific application of a vacuum conveyor is in silk screen printing, where printed sheets travel down a delivery conveyor after printing for ultraviolet ray drying and stacking. The vacuum conveyor is partially situated over a terminal end of the delivery conveyor. Sheets are "taken off" the delivery conveyor by the vacuum conveyor and delivered to a sheet stacking apparatus, such as a vibrating sheet stacker. The sheets are not damaged by this process and are delivered in a more controlled manner than would be the case if they were simply allowed to freely discharge from the end of the delivery conveyor.
Heretofore, insufficient control over the amount of suction present in the vacuum conveyor has been maintained. Most systems rely on the suction pressure to vary merely by the distance of the conveyor inlet apertures from the vacuum source. That is, the suction is greatest near the source and lessens as the conveyor extends away from the source. To initiate disengagement of the sheet or other article, the systems rely on the vacuum being low enough to allow passive release of the sheet, without much control over where that occurs.
Other systems provide for separation by sealing a section of the vacuum chamber so that air flow through the belt is stopped. Still others provide for active means, e.g., puff-off devices, to encourage separation by blowing air at the sheet through the conveyor belt.
Existing systems generally do not provide for variation of the flow of air within the vacuum chamber to increase or decrease the suction pressure as needed for the specific article to be conveyed. For example, curly and flimsy sheets of material require a relatively high vacuum to hold them to the conveyor without damage. It is also desirable to have a relatively higher suction pressure at the engagement position of the sheet to the vacuum conveyor to ensure that the sheet is properly controlled.